1. Field of the Invention
The present invention relates to a process for rectificatively separating a liquid II whose content of acrylic acid and/or methacrylic acid, based on the total weight of the liquid II, is at least 10% by weight, and which, in addition to methacrylic acid and/or acrylic acid, comprises both acrolein and/or methacrolein and acetone in a total amount of not more than 5% by weight, based on the amount of acrylic acid and/or methacrylic acid present in the liquid II, with the proviso that the weight ratio of acrolein present in the liquid II to acetone present in the liquid II is different from 3.5, and which has been generated without adding acrolein or methacrolein as a pure substance to another liquid I comprising acrylic acid and/or methacrylic acid.
2. Description of the Background
Acrylic acid and methacrylic acid, either as such or in the form of their esters, are of significance especially for the preparation of polymers for a wide variety of applications, for example use as adhesives, use as water-superabsorbent materials, and have a high tendency to free-radically polymerize especially in the liquid phase. Safe storage of liquids comprising acrylic acid and/or methacrylic acid is possible at low temperatures only with addition of polymerization inhibitor. Acrylic acid and methacralic acid are obtainable by methods including partial heterogeneously catalyzed gas phase oxidation of precursor compounds such as alkanes, alkanols, alkenes and/or alkenals which comprise 3 or 4 carbon atoms (or from a mixture thereof). Acrylic acid is obtainable advantageously by partial heterogeneously catalyzed gas phase oxidation of propane, propene and/or acrolein. Methacrylic acid is obtainable advantageously by partial heterogeneously catalyzed gas phase oxidation of tert-butanol, isobutene, isobutane, isobutyraldehyde and/or methacrolein. However, conceivable starting compounds are also those from which the actual C3/C4 starting compounds is only formed as an intermediate during the partial heterogeneously catalyzed gas phase oxidation. An example is the methyl ether of tert-butanol.
The starting gases mentioned are generally diluted with inert gases such as nitrogen, CO2, other saturated hydrocarbons, CO, noble gas (He, Ar, etc.) and/or steam, passed in a mixture with oxygen, at elevated temperatures (typically from 200 to 400 or to 450° C.) and, if appropriate, pressure elevated above standard pressure, over preferably transition metal mixed oxide catalysts (comprising, for example, Mo, Fe and Bi or Mo and V or Mo and P) and converted oxidatively to acrylic acid and/or methacrylic acid (cf. for example, DE-A 44 05 059, EP-A 253 409, EP-A 092 097, DE-A 44 31 949, EP-A 990 636, EP-A 1 106 598, EP-A 1 192 987, EP-A 529 853, EP-A 1 350 784, DE-A 198 55 913 and DE-A 101 01 695 and also the prior art cited in these documents).
In general, acrylic acid is prepared from C3 precursors separately from the preparation of methacrylic acid which is prepared starting from C4 precursors. When the starting material is a mixture of C3 and C4 precursors, acrylic acid and methacrylic acid can, though, also be prepared in a mixture by partial catalytic oxidation in the gas phase.
However, owing to numerous parallel and subsequent reactions proceeding in the course of the heterogeneously catalyzed partial gas phase oxidation and owing to the inert diluent gases also to be used and the impurities present in the crude alkanes, alkanols, alkenes and/or alkenals (cf., for example, DE-A 102 45 585, WO 01/96270 and WO 03/011804 (for example use of polymer-grade or of chemical-grade propylene)), not only acrylic acid and/or methacrylic acid are obtained in the heterogeneously catalyzed partial gas phase oxidation, but rather a reaction gas mixture which, in addition of acrylic acid and/or methacrylic acid and the inert diluent gases, comprises by-products from which the acrylic acid and/or methacrylic acid desired as the target product have to be removed.
Typically, the acrylic acid and/or methacrylic acid are removed from the reaction gas mixture of the heterogeneously catalyzed partial gas phase oxidation by initially converting the acrylic acid and/or methacrylic acid from the gas phase into the liquid phase in a basic removal.
This can be effected, for example, by subjecting the product gas mixture of the heterogeneously catalyzed partial gas phase oxidation to a condensation step which can be induced by direct and/or indirect cooling. Preference is given to performing the condensation step in a fractionating manner (cf., for example, DE-A 199 24 532, DE-A 199 24 533, DE-A 197 40 253, DE-A 196 27 847 and DE-A 103 32 758).
Alternatively, the acrylic acid and/or methacrylic acid can also be converted to the condensed phase by absorbing them out of the reaction mixture of the heterogeneously catalyzed partial gas phase oxidation into a suitable absorbent (cf., for example, US 2004/0242826, DE-A 196 06 877, DE-A 196 31 645, EP-A 982 289, EP-A 982 288, EP-A 982 287, EP-A 792 867, EP-A 784 046, DE-A 103 36 386, DE-A 43 08 087, DE-A 21 36 396, EP-A 648 732 (especially the working example), EP-A 1 125 912, EP-A 1 212 280).
Useful absorbents are, for example, water, aqueous solutions and organic solvents. Preferred organic absorbents are those whose boiling point under standard conditions (1 bar) is above the boiling point of acrylic acid and/or methacrylic acid and which are preferably comparatively hydrophobic in order to very substantially prevent coabsorption of water of reaction.
Such suitable organic absorbents are, for example, a mixture of from 70 to 75% by weight of diphenyl ether and from 25 to 30% by weight of diphenyl and also mixtures of from 0.1 to 25% by weight of o-dimethyl phthalate and from 75 to 99.9% by weight of an aforementioned diphenyl ether/diphenyl mixture (cf., for example, DE-A 43 08 087 and DE-A 21 36 396).
It will be appreciated that absorption and condensation may also be employed in combination, as described, for example, by EP-A 784 046 and by DE-A 44 36 243.
Depending on the aim, it is possible to remove low-boiling secondary components from the condensate or absorbate by means of desorption and/or stripping by means of gases such as nitrogen or air. Subsequently, the acrylic acid and/or methacrylic acid can be removed in any purity from the remaining condensed phase via rectificative separating sequences. It will be appreciated that the rectificative removal of the acrylic acid and/or methacrylic acid may also be undertaken directly out of the absorbate or condensate. When the conversion from the gas phase into the condensed phase is carried out in such a way that it comprises the acrylic acid and/or methacrylic acid in comparatively dilute form (for example only to an extent of 30% by weight, or only to an extent of 20% by weight, or only to an extent of 10% by weight), the prior art also recommends enriching the acrylic acid and/or methacrylic acid out of the directly obtained condensed phase into a suitable extractant by extraction by means of said extractant (before or after a desorption and/or stripping) (cf., for example, patent 54 354 of the German Democratic Republic and EP-A 312 191) and subsequently undertaking the rectificative removal of acrylic acid and/or methacrylic acid starting from the extract.
When an aqueous phase has been used as the absorbent, a first rectificative separation step may also consist in removing water from the condensed phase with the aid of an azeotropic entraining agent (cf., for example, US 2004/0242286, EP-A 695 736, EP-A 778 255, EP-A 1 041 062, EP-A 1 070 700).
A disadvantage of the procedures described is that the reaction gas mixture of the heterogeneously catalyzed gas phase partial oxidation essentially unavoidably still comprises residues of the precursor aldehyde of acrylic acid and/or methacrylic acid, i.e. acrolein and/or methacrolein. This can be attributed to the fact that the heterogeneously catalyzed catalytic oxidative preparation of acrylic acid and/or methacrylic acid in the gas phase always proceeds via the corresponding precursor aldehyde as the precursor (cf., for example, JP-A 7-10802).
This is disadvantageous in that, according to DE-A 195 39 295 for example, the polymerization tendency both of acrylic acid and of methacrylic acid is significantly increased in the presence even of the smallest amounts of their precursor aldehyde (in the ppm range). This is disadvantageous in particular because the above-described condensed phases comprising acrylic acid and/or methacrylic acid normally in general also comprise small amounts of the corresponding precursor aldehyde (depending on the contamination of the raw material used for the partial oxidation, both precursor aldehydes, i.e. acrolein and methacrolein, may be present both in the case of acrylic acid and in the case of methacrylic acid), which is one cause of the fact that, under the thermal stress of a rectification in particular, the rectificative separation of liquids which, in addition to methacrylic acid and/or acrylic acid, also comprise acrolein and/or methacrolein, has to be interrupted from time to time owing to undesired polymer formation.
As a countermeasure, the prior art recommends carrying out both the conditions of the heterogeneously catalyzed gas phase partial oxidation and the conversion of the acrylic acid and/or methacrylic acid from the gas phase into the liquid phase and the treatment of the latter in advance of a subsequent rectificative separation in such a way that the liquid which is to be treated rectificatively and comprises acrylic acid and/or methacrylic acid comprises a minimum amount of acrolein and/or methacrolein and a minimum amount of other aldehydic by-products, for example formaldehyde, glyoxal, acetaldehyde, propionaldehyde, n-butyraldehyde, isobutyraldehyde, furfural, benzaldehyde and crotonaldehyde (all of which equally have polymerization-promoting action; however, the precursor aldehydes (also owing to their physical properties) are of the greatest significance with regard to amount and effect). Influencing parameters to be selected suitably in this regard are the selection of the multimetal oxide catalysts, the selection of the raw material, the selection of the reaction temperature, the selection of the conversion, the selection of the degree of dilution in the reaction gas mixture, the selection of the absorbent, the selection of the absorption conditions, etc.
All of the aforementioned measures are afflicted with disadvantages. In particular, pure raw materials are comparatively expensive.
High conversions of the aldehydic precursor compound to the acrylic acid and/or methacrylic acid target products are typically necessarily associated with an increased proportion of full combustion to CO2 and H2O. Reduction in the aldehyde content brought about by desorption and/or by means of low boiling stripping normally causes losses of acrylic acid and/or methacrylic acid. Highly selective multimetal oxide catalysts are generally comparatively costly and inconvenient in their preparation, etc.
Alternatively, the prior art recommends chemically binding the aldehydic secondary components by a rectificative pretreatment by means of compounds which have an amine group (cf., for example, EP-A 312 191, DE-A 22 07 184, DE-A 195 39 295, EP-A 270 999, DE-A 196 34 614, etc.).
However, a disadvantage of this procedure is that it requires an additional material. EP-A 1 041 062 for the first time addresses the role of a further secondary component which is unavoidably also formed in the catalytic oxidative preparation in the gas phase of acrylic acid from C3 precursor compounds (in particular propylene, acrolein and/or propane) and in the catalytic oxidative preparation in the gas phase of methacrylic acid from C4 precursor compounds (in particular isobutene, methacrolein, tert-butanol, isobutyraldehyde, isobutane and/or the methyl ether or tert-butanol). This secondary component is acetone.
According to the teaching of EP-A 1 041 062 and the accompanying opposition file, the acetone is postulated to have the same action as the precursor aldehydes acrolein and methacrolein. In other words, it is assumed that presence of acetone undesirably promotes the tendency of acrylic acid and/or methacrylic acid to free-radically polymerize in the same way as presence of acrolein and/or methacrolein in the condensed phase. In this regard, even synergistic interaction of acetone and the precursor aldehydes is assumed.